DL-Methionine and L-Methionine are Equally Efficient in Broilers

Commercial poultry diets are routinely supplemented with methionine (Met) sources to precisely meet their Met+Cys specifications. Globally, dry DL-methionine (DL-Met, 99% purity) is the most commonly used Met source followed by methionine hydroxy analogue products (MHA-FA liquid, 88% purity and dry MHA-Ca, 84% purity) and L-methionine (LMet, 99% purity). During recent years, numerous studies designed to determine the replacement ratio of L- and DL-Met products have shown that L-Met and DL-Met are 100% equally efficient in broilers (Baker 1994; Ribeiro et al., 2005; Baker 2006; Dilger and Baker, 2007). Although the 100% nutritional equivalence of L- and DL-Met has been well documented, few other publications claimed higher bioavailability for L-Met (Shen et al. 2014; Park et al., 2017). Furthermore, a new L-Met source with a minimum content of 90% L-Met has been introduced to the market claiming that it can replace DL-Met (99% purity) on 1:1 product-to-product basis because of its higher bioavailability. Therefore, a study was conducted to determine the bioavailability of L-Met90 compared to diluted DL-Met to 90% purity in male broilers from 1 to 34 d of age.

A total of 1,800 d-old male Ross 308 broilers were allocated to 90 floor pens of 20 broilers each. Each pen (~ 3 m²) was equipped with a bell drinker and a round feeder. Light and temperature regimes were managed according to the breeder's recommendations and complying with EU welfare legislation. Broilers were fed a 3-phase feeding schedule with starter (d 1 to 10), grower (d 11 to 26), and finisher phases (d 27 to 34). Starter feeds were produced in crumbles while grower and finisher diets (3.0 mm) were steam pelleted. Feed and water were supplied ad libitum throughout the experimental period. Each phase comprised 17 treatments including a basal diet deficient in SID Met+Cys without supplemental Met, and 8 increasing levels of either L-Met90 or DL-Met diluted to 90% purity – DL-Met90. Starch was used to dilute DL-Met to a Met content of 90%. Met sources were added in all phases on weight-to-weight basis at: 0.00, 0.30, 0.60, 0.90, 1.20, 1.50, 2.10, 2.70 and 3.60 g/kg. There were 10 replicate pens for the basal treatment and 5 replicate pens for each of the Met supplemented treatments (2 to 17). Diets were formulated to meet or exceed amino acid recommendations by AMINOChick® 2.0, except for SID Met and Met+Cys. Main ingredients were analyzed by AMINONIR® and results were used for diet formulation. In addition, Met sources were analyzed for Met purity. The SID Met+Cys levels of the starter, grower, and finisher basal diets were 6.1, 5.3 and 5.1 g/kg, respectively, as shown in Table 1. Growth performance variables were evaluated for each feeding phase. On day 34, four birds close to average pen weight were selected for carcass evaluation. After cooling, carcass yields (CY) were determined, prior to fileting for breast meat yield measurement (breast without skin and bone). Growth performance and carcass data were subjected to multi-exponential regression analysis using the nonlinear-regression procedure described by Littell et al. (1997). The significance of the RBV estimate was defined by the values of the 95% confidence intervals. No significant difference between L-Met90 and DLM90 is declared if the approximate 95% confidence interval includes the value of 100%, whereas a confidence interval completely above 100% would indicate statistical superiority of L-Met90.

Analyzed values of the experimental diets from all dietary phases were in close agreement with the calculated values (Table 1). The commercial lot of L-Met90 used in the study was analyzed to contain a purity of 93%, which is greater than its minimum guarantee content of 90% L-Met. Therefore, the analyzed values for both supplemental Met sources (DL-Met90 and L-Met90) for each phase were used to calculate the total intake of supplemental Met and used for RBV determination.

Increasing levels of either L-Met90 or DL-Met90 significantly improved growth performance and carcass yields compared to the basal diet. Relative to the basal diet, the highest Met addition improved body weight gain (BWG) by 79 and 76% and reduced feed conversion ratio (FCR) by 22 and 21% for DL-Met90 and L-Met90, respectively. Similarly, CY and breast meat yield as percentage of carcass weight were improved by 7 and 7% and 51 and 51% for DL-Met90 and L-Met90, respectively. These relative responses demonstrate that the basal diets were clearly deficient in dietary Met+Cys, and that both Met sources showed a common plateau at the highest Met addition. Therefore, data fit well for multi-exponential regression analysis for RBV determination

Overall responses (d 1 to 34) to increasing levels of DL-Met90 and L-Met90 are shown in Figures 1 and 2 for growth performance and carcass yield, respectively. Multi-exponential regression analysis revealed that L-Met90 was 106 and 96% as efficacious as DL-Met90 for BWG and FCR, respectively. Based on carcass traits, L-Met90 was estimated to be 86, and 93% as efficacious as DL-Met90 for CY and breast meat weight as percentage of carcass weight, respectively. However, based on the confidence intervals none of the RBV estimates were significantly different from 100%.

No significant differences were found for any of the RBV estimates of L-Met90 compared to DLM90 for any of the feeding periods. These results demonstrate that the new LMet source with a minimum purity of 90% L-Met is equally efficient as a diluted DL-Met to the corresponding purity of 90% in broilers from 1 to 34 d of age. These results are consistent with previous reports (Baker 1994; Ribeiro et al., 2005; Baker 2006; Dilger and Baker, 2007) that have demonstrated the same nutritional value of L-Met (99%) and DL-Met (99%) in broilers.